1. Field of the Invention
The present invention relates to processing equipment for purifying fluids, namely, filtering apparatus.
The invention may be particularly advantageous when purifying natural gas by separating mechanical impurities, liquid hydrocarbon admixtures, and water, dispersed in the gas. Also, the apparatus of the present invention may be utilized for purifying and separating liquids, such as oils and fuels.
The degrees of purification in gases and liquids at high flow rates is a relatively long standing problem, yet experience has not shown any satisfactory solution thereof. Recognition of this problem as being currently urgent can be seen by a great number of patents which have issued from 1965 to 1978 in different countries. Furthermore, the prior art fails to ensure 100% separation of disperse mechanical impurities and mists of liquids from gases at a high rate of flow, despite the fact that a demand for such ensured and full purification is continuously increasing with the development of nuclear engineering.
2. Description of the Prior Art
Principally two approaches to solving the problem were repeatedly attempted heretofore. It was 15 years ago that in some countries a filter comprising a hollow casing having an inlet and an outlet was used for purifying small volumes of natural gas at a low rate of flow. Contained within the casing was a filter insertion piece made of a porous material. Specifically, the filter insertion piece was made of a porous ceramic material due to an appreciable pressure of the gas being filtered. To the best of the applicant's knowledge, such filters have not been used on gas fields for a number of reasons due to disadvantages inherent in such an arrangement.
The most significant drawback of the structure in question is a frequent clogging of the filter insertion piece by solid particles and by sedimentation of heavy fractions from hydrocarbon condensate, which leads to the build-up of a pressure differential in advance of and behind the filter insertion piece, giving rise to unsafe stresses in the porous material. The filter insertion pieces would have been to be frequently replaced, and to provide for such a replacement, stand-by filtration systems would have to be designed and built. It is likewise noted that even the most effective filter cleaning methods presently employed do not provide for completely restoring the original characteristics of the filter material.
A further significant drawback of the filters making use of filter insertion pieces of a porous material is that the filtration process has to be a multistage one i.e., there have to be filter insertion pieces for both fine and coarse purification.
It will be understood by those skilled in the art that such an array of filter insertion pieces displays too great a resistance to dynamic loads and limits the output of such a filtration process (well yields in gas fields).
A different approach to solving the problem consisted in using gravity and centrifugal forces for separating mechanical and other impurities from the fluid to be filtered. Thus, the prior art includes the use of such centrifugal separators as disclosed in U.S. Pat. No. 3,201,924 and French Pat. No. 1,252,017. The apparatus of the type mentioned above comprises a hollow casing, an impeller or helical bore used as a spinner, and an expansion chamber. The gas to be purified is set in rotary motion, whereby mechanical impurities and small globules of suspended liquid under the action of centrifugal forces are thrown against the walls of the casing and trapped in the chamber.
An obvious advantage of such a purification system or arrangement is in the higher output of the purification process, a wholly satisfactory hydraulic resistance produced by the filtering apparatus and its being convenient in operation. However, as to a quality characteristic of the purification process carried out on the centrifugal separators, it is worse than that of filtering apparatus having filter insertion pieces made of a porous material. The centrifugal separators hardly separate particles of 100 .mu.m when purifying gases, as has been shown by experimental data.
Known in the art is an attempt to combine the advantages of both the above types of separators as disclosed in U.S. Pat. No. 2,198,819.
The specification together with the drawings teaches the abovementioned apparatus which comprises a hollow cylindrical casing, a spinner, a hollow filter element and a discharge connection piece. The casing is provided with an inlet and an outlet. The cylindrical filter element is positioned coaxially within the casing and is fit with its open end in the outlet. The above filter element comprises a screen frame with a filter material made as a metal gauze.
In operation, the vanes of the spinner impart rotary motion within the casing to the flow of the fluid being purified. In doing this a portion of the foreign matter under the action of the centrifugal force is thrown against the walls of the casing and is discharged through the discharge connection piece, while the other portion, made up of fine mechanical particles and suspended moisture, is separated by the filter material. Consequently, such filtering apparatus is capable of a relatively prolonged operation without the need to replace the filter element, and a high output of the purifying process is ensured.
But even this apparatus, though showing the above-mentioned advantages, does not always meet quality requirements.
Entry of the fine particles into the filtered fluid, among other things, is not excluded, and is increasingly probable as the filtering element gets clogged, and is under the constant action of the turbulent upper boundary layer of the fluid being purified.
Besides, replacement of the filter elements is nevertheless imperative, but this presents considerable difficulties particularly in the case of separating radioactive dust from gases, for example.
To eliminate any clogging of a filter element was the object of an inventive apparatus for "wet" purification of gases as disclosed in U.S.S.R. Author's Certificate No. 575118. Natural gas and a number of other gaseous media are known to contain a suspended liquid which under specific conditions can form a thin film on the walls of the casing and of the filter element, which film permits gas to pass through the filter element, but precludes entry or stands in the way of disperse particles.
To utilize this principle of purifying, the apparatus involved, comprising a hollow casing, a spinner and a hollow filter element, was provided with the hollow filter element formed as a cylindrical frame covered with a fluorine plastic. The spinner was described as positioned at the inlet within the casing, and the open end of the filter element fit in the outlet. The casing and the filter element were coaxially positioned cylinders equally spaced along the length of the latter.
A fluorine plastic covering, being water-repellent with a dynamically smooth surface, was conceived to provide a protective liquid film over the outer surface of the filter element when the gas-filtrant revolves within the casing, and at the same time prevent ingress of the film into the pores. As the film grew in thickness, the liquid, while breaking away from the fluorine plastic covering, was to carry separated disperse particles and liquid impurities to the circumference of the casing. The filter element could not clog and this would be clearly a solution of the problem.
But regardless of the concept being accurate and promising, investigation and tests have shown that disperse mechanical particles, hydrocarbon condensate and moisture were blocked from penetrating the pores of a water-repellent material only over that portion of the filter element which is adjacent the inlet. Moreover, an analysis of the gas-filtrant has shown the presence of hydrocarbon condensate and moisture therein.